Currently, all potassium hydroxide plants in the United States sell liquid chlorine and potassium hydroxide. Chlorine and potassium hydroxide are transported by rail to customers. At present, five plants produce chlorine and potassium hydroxide. Potassium chloride and water are electrolyzed to produce chlorine, potassium hydroxide and hydrogen according to the following reaction:2KCl+2H2O-->Cl2+2KOH+H2 Between 2,400 and 3,200 kiloWatt hours per ton of chlorine produced are needed for this reaction depending on the electrolysis technology.
Chlorine, potassium hydroxide and hydrogen are produced from an aqueous potassium chloride solution by mercury, asbestos diaphragm or membrane cell electrolysis. With some variations, the usual process involves brine purification and de-ionization, electrolysis, chlorine processing, potassium hydroxide processing, and hydrogen processing.
Processes for potassium hydroxide (KOH) production are well known in prior art. One such process is depicted in FIG. 1. Solid potassium chloride (KCl) salt 1 arrives at a processing plant via rail car 4. Potassium chloride 1 is transferred to a salt dissolver 2, wherein the solid, impure potassium chloride 1 is dissolved in water to form brine 3. At some plants, sodium chloride (NaCl) is substituted for potassium chloride to ultimately produce sodium hydroxide (NaOH) instead of potassium hydroxide (KOH). Next, appropriate chemicals are added to the brine 3 to precipitate impurities. The resulting mixture is fed to a thickener 5, from which precipitates and clarified raw brine 3 are separately withdrawn. The clarified brine 3 is then filtered by brine filter 6. Solid precipitates are sent to a landfill, or disposed of by any suitable method. The filtered brine 3 is then pumped to a filtered brine tank 7. Next, secondary brine purification is performed on brine 3 via an ion exchange unit 8. Secondary brine purification ensures high efficiency and long lasting membrane electrolyzer operation. Deionized brine 3 is stored in tank 9 before being sent to electrolyzer 10.
In electrolyzer 10, electric power is provided via an AC-DC rectifier 11. Deionized brine 3 and purified water 12 are pumped into electrolyzer 10. Application of electricity causes anions, i.e. chloride ions, to collect at the anode side of the electrolyzer 10 and cations, i.e. potassium and hydrogen ions, to collect at the cathode side of the electrolyzer 10. The chlorine produced from the weak solution of brine 3, is either drawn from the anode side of the cell in a vacuum or the solution is pumped to a dechlorination process.
To process the wet chlorine, the gas is cooled and the brine 3 condensate is removed in a dechlorinator 14. The weak solution of brine 3 is then returned to the brine treatment area, specifically the salt dissolver 2. Next, the chlorine gas is dried with a sulfuric acid dryer 15. The dry gas is then compressed and chilled for storage in rail cars 17. The chlorine gas may also be stored in cylinders or bulk plant storage.
However, drying, compressing storing the chlorine gas are major drawbacks to plant operation. Drying and compressing the chlorine is expensive and increases the capital costs for plant operation by as much as 10-20%. Additionally, storage of chlorine gas on-site is undesirable because it is potentially very hazardous and may be a terrorist target.
Potassium hydroxide and hydrogen, collected at the cathode side of electrolyzer 10, are drawn from the membrane cell electrolyzer 10. Potassium hydroxides leave the electrolyzer 10 at approximately 30-35% by weight in an aqueous solution at a temperature between approximately 190 and 200° F. This low strength potassium hydroxide solution is split. One stream is cooled and stored for use in the brine treatment tank 5. The other, major, stream is sent to the evaporator 18 for removing the water. In the evaporator 18, the hot potassium hydroxide solution is concentrated to the commercial grade specification, i.e., approximately 45%. Next, the product is cooled to about 170° F. and sent to product storage 20.
Hydrogen gas, also produced on the cathode side of the electrolyzer 10, may be sent to vent 21 for pressure control. The majority of the produced hydrogen, however, is cooled to remove water vapor before it is pumped to hydrogen compressor. After the hydrogen is compressed, it is sent to the boiler 22 where it is burned as fuel.
Currently, there are many chemical plants in the United States that produce sodium hypochlorite by the chemically reactive combination of chlorine and sodium hydroxide. However, current facilities require the delivery of hazardous chlorine gas by truck or rail to the production site. Integration of potassium hydroxide production with bleach manufacture is a logical combination that can both reduce transportation costs and eliminate the need to store hazardous chlorine gas on-site. Presently, however, there are no known plants in the United States that produce potassium hydroxide and sodium hypochlorite by the processes according to the present invention.
It is anticipated that there will be a new demand for these dedicated chlorine plants, since the Department of Homeland Security wants the public transportation of hazardous chlorine and chlorine storage eliminated as possible terrorist targets.